Jim visits his old teacher, Professor Robey, who claims ignorance when Jim explains that he came there when he wrote a letter requesting help. Robey's hand shakes as he pours out drinks, and Jim realizes that he's in trouble. The professor initially tries to deny it, and then gives in and shows Jim a photo of a beautiful woman. Robey explains that her name is Lilith, and he met her when he was supervising Sumerian excavations. He fell in love with her, but discovered that she was evil. There's a knock on the door and one of Robey's students, Robert Perry, comes in. Robert helps Robey on with his robes, and the professor asks Jim to wait until after his meeting. Once Robey leaves, Robert notes that something has been bothering him. He has no idea who Lilith is. Her photo suddenly bursts into flame, and they hear Robey screaming outside. They run to the window and watch helplessly as Robey burns up.
A week later, Jim meets with his supervisor, Colonel Fairchild, who suggests he use some of his accrued leave. Jim isn't interested, and Fairchild warns that the Bureau is answerable to Capital Hill. He officially denies Jim's request, and then unofficially tells him that the chairman of the committee, Senator Waterford, is responsible. Jim goes to see Waterford and asks why he was taken off of the Robey case. Waterford tells him that they have to stick with Federal matters. He invites Jim to stay, but the agent says he has official business to tend to Jim starts to leave and Waterford's wife, Astarte, comes in. He introduces her as Astarte, but Jim recognizes her as the woman in Robey's photograph. Jim hints that he knows about her connection to Robey, and agrees to stay for dinner after all.
Waterford takes Jim to the dining room where a magician, Asmodeus, is practicing. Also present is Dr. Tristam, Waterford's physician. Asmodeus says that Tristam is a skeptic, and the doctor dismisses Asmodeus' magic as trickery. The magician makes Tristam's watch disappear and then descend from the ceiling, only to vanish again and appear in his pocket. Waterford leaves for his annual physical checkup and Asmodeus departs shortly thereafter. Astarte invites Jim out into the garden and he asks how she persuaded Waterford to remove him from the Robey case. She readily admits that she did, and Jim admits that he can't tell Waterford because he's smitten. Astarte invites him to sit down with her and starts to kiss him. Asmodeus tries to ambush him, and Jim easily knocks him down. Artie arrives and Jim bids them farewell. As the agents leave, Jim has Artie check the files on Asmodeus. He gets into a carriage, and the driver pulls a lever. After a moment, he looks inside and confirms that Jim is gone. As he leaves, a man with a bandaged face wearing Jim's suit gets into an ambulance.
Later, Artie is called to the hospital where he's informed that Jim received massive head injuries in an accident. As he sits at the bedside, Colonel Fairchild arrives.
Jim wakes up in a room with only a hatchway some 30' above as the only exit. A projection of Asmodeus appears to taunt the agent. Once it disappears, the metal walls begin to heat up. Jim removes a hidden charge from his boot heel and places it beneath a discarded bucket. Using a piece of wood touched to the wall, Jim lights the fuse and the shaped charge blasts him up to the hatchway. He climbs up and hears chanting nearby. Investigating, Jim finds Astarte leading a Satanic ceremony of hooded figures. She takes out a dagger and prepares to stab a bundled figure on the altar. Jim runs in... and Asmodeus ambushes him from behind.
Artie receives a message from Jim saying to meet at Waterford's manor. Waterford, Fairchild, and Tristam have received similar messages. Fairchild explains that Jim disappeared from the hospital a year ago. Outside, the cab arrives and the driver escorts a stunned Jim to the door. The butler brings him inside and Astarte and Asmodeus arrive. Jim accuses them of presiding at a witch's sabbath, but Tristam concludes that he's been drugged. Artie explains to Jim that he's been in the hospital for the last 12 hours.
Later, Jim is at the train packing for a mandatory furlough. Artie realizes he's telling the truth, and explains that a number of distinguished scientists have met with mysterious and fatal accidents. In each case, Asmodeus or Astarte were nearby. The agents go to Fairchild and convince him to open one of the scientist's graves, although they're not sure what they'll find. The gravediggers remove the coffin and open it, and Fairchild confirms the remains belong to the scientist in question. He tells Jim to take his furlough and leaves.
As Jim and Artie go, Jim hears a humming noise and follows it to a nearby mausoleum. They enter and descend a stairway to the room where Jim was held. Jim continues on to the chapel where the chanting noise is coming from. The room is filled with hooded figures, but the agents realize they're dummies. The "chanting" is provided by a gramophone. Asmodeus reveals himself and orders them to surrender at gunpoint. He almost reveals his plan, but restrains himself at the last minute and tells them that there's much more. Asmodeus lets slip that he is working for a man, but then orders them to a wall and tells them to manacle themselves. Artie does so, but Jim throws a plaque at Asmodeus. They both grab swords, but Jim discovers his has transformed into a bouquet. Outmatches, Asmodeus dives into a trunk and disappears, only to run at Jim with a spear. Jim finally knocks him down and Asmodeus falls on his own spear, dying.
As Jim frees Artie, a door in the wall opens. Inside is Dr. Tristam, who invites them in. He warns them to move carefully, and then admits that he created devices like the variant of Greek fire that he used to kill Robey. When an angry Artie goes for him, Tristam stuns him unconscious with electricity. The doctor then admits that Jim's theory that he's using the scientists for his own means is correct. He opens another panel and displays vats, each one containing the brain of a dead scientist. Tristam explains that he's using them to cogitate on his behalf, and he plans to collect many more. When Jim claims skepticism, Tristam tortures one of the brains to perform a calculation. he then shows Jim the central collector of the brains' power. Astarte has collected the scientists, but they were forced to kill Robey when he grew suspicious. Now she's seduced Waterford for his political contacts, but she's gotten restless. At midnight, Waterford will wear the cloak that Astarte gave him and it will combust on queue.
Tristam warns Jim not to move, and he notes that the brains must hate Tristam. As the vats loudly begin to bubble, Jim suggests that they make one collective effort. The brains release a burst of electricity, setting Tristam on fire as Jim gets Artie out. He leaves Artie outside and heads for Waterford's manor.
At the manor, Jim finds Astarte in the garden. She flirts with him again, but is shocked to learn that Asmodeus and Tristam are dead. When Waterford comes out, Astarte claims that Jim assaulted her. The senator draws a gun and vows to shoot Jim dead, and the agent tells him to get out of the cape. He distracts Waterford long enough to pull the cape off and wrap it around Astarte. Rather than face a hideous death, she breaks free. The cape ignites and Waterford walks away, realizing the truth. Jim promises Astarte that she'll hang from the highest gallows.
Later, Artie is practicing magic at the train. When Jim comes in, Artie demonstrates his ability to produce a woman from an empty cabinet. She immediately kisses Jim, and Artie admits a rabbit was supposed to appear.